Halo gas cross sections and covering fractions of MgII absorption selected galaxies

We examine halo gas cross sections and covering fractions, f(c), of intermediate-redshift Mg II absorption selected galaxies. We computed statistical absorber halo radii, R-x, using current values of dN/dz and Schechter luminosity function parameters, and have compared these values to the distribution of impact parameters and luminosities from a sample of 37 galaxies. For equivalent widths W-r(2796) >= 0.3 angstrom, we find 43 <= R-x <= 88 kpc, depending on the lower luminosity cutoff and the slope, beta, of the Holmberg-like luminosity scaling, R proportional to L-beta. The observed distribution of impact parameters, D, are such that several absorbing galaxies lie at D > R-x and several non-absorbing galaxies lie at D < R-x. We deduced that fc must be less than unity and obtain a mean of < fc > similar to 0.5 for our sample. Moreover, the data suggest that halo radii of Mg II absorbing galaxies do not follow a luminosity scaling with beta in the range of 0.2-0.28, if f(c) = 1 as previously reported. However, provided f(c) similar to 0.5, we find that halo radii can remain consistent with a Holmberg-like luminosity relation with beta similar or equal to 0.2 and R-* = R-x/root f(c) similar to 110 kpc. No luminosity scaling (beta = 0) is also consistent with the observed distribution of impact parameters if f(c) <= 0.37. The data support a scenario in which gaseous halos are patchy and likely have non-symmetric geometric distributions about the galaxies. We suggest that halo gas distributions may not be governed primarily by galaxy mass/luminosity but also by stochastic processes local to the galaxy.